The M4 mortar carrier was based on the half-track car M2, and the mortar was placed in the rear compartment and pointed rearwards. The side ammunition compartments were retained, and additional 81mm mortar bomb stowage compartments were placed behind the standard stowage boxes. A rear door was installed on the M4, but the machine gun skate mount was retained, forcing crewmen to crawl under the skate rail if they wanted to use the rear door. The mortar was intended to be fired from the ground except for emergency situations, so the vehicle mortar mount was very limited in traverse.

The limited traverse mount of the M4 was unacceptable once it was realized that it was desirable to fire the mortar mounted on the carrier. The M4A1 incorporated a new rough traverse mechanism that allowed 600 mils of traverse in 100-mil increments, but fine adjustments were still made with the mortar's bipod mount. To accommodate the rough traverse feature, the mortar socket was elevated 7.375" (18.73cm) and the rear mortar bomb ammunition racks were removed. Ammunition stowage remained at 96, however. The M4A1 also featured stowage boxes mounted on the outside of the rear plate, on either side of the access door.